BEAUTY: 
BRAGGING RIGHTS: A beautiful sight
HOW EASY IS IT TO SEE? Requires special equipment
TYPE: Special event
DISCOVERED: Known since antiquity
Most maps of the solar system are two-dimensional and represent the orbits of the planets by concentric circles. This is a reasonable approximation because planetary orbits are mostly aligned on a plane, known as the ecliptic. Similarly, when you see a diagram of the moon causing eclipses, you see it on a flat plane—the moon goes around the Earth and when it gets in front of the sun, it causes an eclipse. But that poses an obvious question: why don’t solar eclipses happen every time?
The reality, of course, is that the real solar system is three-dimensional. The orbit of the moon around the Earth is almost, but not quite, on the same plane as the ecliptic. In truth, the moon’s orbit is inclined by five degrees with respect to the Earth’s orbit. Most of the time when the moon gets in front of the sun, it is above it or below it in the sky and no eclipse happens.
Eclipses are all about being at the right place at the right time. Imagine the moon’s orbit inclined 5 degrees to the ecliptic. The orbit crosses the ecliptic at exactly two points. If the sun happens to be in that direction when the moon is at that exact point, then you get a solar eclipse.
And there’s one more factor: the Earth is three-dimensional too. Imagine that we’re witnessing a total eclipse of the sun. The moon is at the ecliptic and the sun is exactly at the right point. But the Earth is a sphere. Imagine moving north on that sphere: we’ll actually be moving above the plane of the ecliptic. If we move far enough, eventually we’ll be out of alignment and we won’t see a total eclipse.
WHAT YOU MIGHT SEE THROUGH AMATEUR EQUIPMENT
If everything aligns perfectly, we get a total eclipse. But if we’re off by a little bit, then we get a partial one. But even partial eclipses of the sun are rare. The sun and the moon have to align, and you have to be in the right place on Earth at the time it occurs or you won’t see anything unusual.
Witnessing even a partial eclipse, we’re reminded of the complex clockwork of the solar system. Even ancient astronomers, who kept track of the sky year after year, could not predict solar eclipses with accuracy. It wasn’t until we discovered the actual geometry of the solar system, and the exact timing and measurements of the orbits, that we could finally predict the heavens.
List of Upcoming Partial Solar Eclipses
• August 21, 2017: Total in parts of the US; partial in most of North America.
• August 11, 2018: Partial in northeastern Canada.
• June 10, 2021: Annular in parts of Canada; partial in the northeastern US.
• October 14, 2023: Annular in western US; partial in most of North America.
• April 8, 2025: Total in central US; partial in most of North America.
• August 12, 2026: Partial in Canada and northeastern US.
Practice solar observing. Many of the tips pertaining to observing total eclipses apply to partial eclipses. Partial eclipses, which happen more often, are a great time to practice your techniques for a full solar eclipse.
Annular eclipses. Even if the moon and the sun are aligned, the moon is sometimes too far away to completely cover the sun. In those cases we get an annular eclipse—the moon leaves a bright ring of sunlight (known as the “ring of fire”) behind. Annular eclipses are beautiful for their symmetry, even if you don’t get full darkness.
Find the crescent moon. During the day of the eclipse, the new moon and sun are aligned. But the moon will continue to move in its orbit, getting further and further away from the sun. At sunset on the day of the eclipse, see if you can find the very beginnings of the crescent moon. If you can’t find it, try again on subsequent days.
